The heat treatment tank of a workpiece used in metal plating treatment and the like uses a steam heat exchanger having a configuration in which a steam heating pipe is disposed on the bottom side of the tank interior, and the workpiece inside the heat treatment tank filled with a treatment liquid is heated. An example of a conventional steam heat exchanger disposed in an open treatment tank is shown in FIGS. 3 and 4.
The steam heat exchanger 100 shown in FIG. 3 is a lift-fitting steam heat exchanger and has a steam heating pipe 103 drawn about in the form of an accordion so as to be two-tiered in the vertical direction in a position near the bottom surface of an open heat treatment tank 102 in which the treatment liquid 101 is held. Steam at a prescribed pressure is supplied from a boiler or another steam supply source 105 to a steam heating pipe 103 by way of a steam supply pipe 104. Heat is exchanged with the treatment fluid 101 by using the latent heat of steam provided through the steam heating pipe 103. Heat-exchanged steam becomes condensed water (saturated water), enters a steam heating pipe 103a on the lower side, and is recovered via this route from the drain conduit 106 by way of a steam trap 107 or another drain discharge device.
Also, the steam heat exchanger 200 shown in FIG. 4 is an example of a steam heat exchanger in which the drain is evacuated from the bottom of an open tank 201 and which does not require lift fittings. This steam heat exchanger 200 is provided with a steam supply port 202 and a steam discharge port 203 on the side of the open tank 201, and the steam heating pipe 204 extends into the tank interior from this location in a horizontal ‘U’ shape. In this case as well, the latent heat of steam that passes through the steam heating pipe 204 is used for exchanging heat with the treatment fluid 205 in the tank.
In this case, the following structures and methods of use are commonly adopted in conventional steam heat exchangers that use the latent heat of steam to perform heat exchange.
(1) Since the large condensation heat transfer rate of steam is utilized, a structure is adopted in which condensation water is smoothly separated from the heat transfer surface, and the heat exchange surface is constantly covered by steam without being submerged.
(2) The discharge capacity is greater than the required condensation rate at the service temperature of the steam heat exchanger with consideration given to the start load of the steam trap, and the discharge capacity is ordinarily double or more than required so that condensation water can be smoothly evacuated from the steam heat exchanger.
(3) The evaporation heat of steam used in heat exchange decreases as pressure increases. For this reason, steam heat exchangers are operated at the lowest possible pressure. As a result, there are cases in which a drain lifter, a vacuum pump, or another drain recovery apparatus is required in condensate recovery.
(4) A preheater that uses drainage evacuated from the steam trap is sometimes provided to increase the heat efficiency of steam heat exchangers. In these cases, only flash steam can be used to prevent water hammering, and it is often the case that the cost-reducing effect is poor because there are limitations to the pressure of a drain recovery pipe.
(5) Since the heat capacity of steam per unit volume is low, two-position control for controlling steam is sufficient in steam heat exchangers in which the start up time is considerable. In positional proportional control, the steam part readily reaches a pressure below the back pressure of the steam trap in addition to undergoing a vacuum phenomenon of the heat exchanger steam part. As a result, smooth drain discharge becomes difficult and there are many cases in which positional proportional control has no significance.